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Exercise 3: Barbituric Acid Dihydrate

Three Unique Molecules​

Initial Analysis​

  1. Open the BARBAD01 CIF
  2. Complete all molecules
  3. For each molecule:
    • Generate Hirshfeld surfaces
    • Map with HF/3-21G electrostatic potential

Surface Analysis​

  1. For each Hirshfeld surface:
    • Rescale surface property to ±0.025 au range
    • Note the planar arrangement of the cluster
    • Extend to create a sheet of hydrogen-bonded molecules using Generate External Fragment
    • Use Clone Surface for each molecule
tip

Clone surfaces separately for each parent surface in the right-hand dialog

info

Observe the persistent pattern of electrostatic complementarity between adjacent molecules

Energy Analysis​

  1. For each molecule:
    • Select Reset Crystal between calculations
    • Select the molecule
    • Create 3.8 Å cluster
    • Calculate energies

Energy Framework Analysis​

  1. Generate unit cell molecules
  2. Create energy frameworks (Display / Energy frameworks):
    • Compare:
      • Coulomb Energy (EeleE_\text{ele})
      • Dispersion Energy (EdisE_\text{dis})
      • Total Energy (EtotE_\text{tot})
    • Observe:
      • Strong interactions between molecules in layers (especially EeleE_\text{ele})
      • Weaker inter-layer interactions (mainly EdisE_\text{dis})
Historical Context

These results help explain the observation "There is marked cleavage parallel to (010)" from the original structure determination by Jeffrey et al.